Method and structure for producing Z-axis interconnection assembly of printed wiring board elements

ABSTRACT

A method of forming a core for and forming a composite wiring board. The core has an electrically conductive coating on at least one face of a dielectric substrate. At least one opening is formed through the substrate extending from one face to the other and through each conductive coating. An electrically conductive material is dispensed in each of the openings extending through the conducting coating. At least a portion of the surface of the conductive coating on one face is removed to allow a nub of the conductive material to extend above the substrate face and any remaining conductive material to thereby form a core that can be electrically joined face-to-face with a second core member or other circuitized structure.

This application is a divisional of application Ser. No.09/834,281,filed Apr. 12, 2001 now U.S. Pat. No. 6,465,084,for METHODAND STRUCTURE FOR PRODUCING Z-AXIS INTERCONNECTION ASSEMBLY OF PRINTEDWIRING BOARD ELEMENTS.

BACKGROUND INFORMATION Field of the Invention

This invention relates generally to a method and structure for producinga Z-axis interconnection of printed wiring board elements and, moreparticularly, to a method and structure of providing a printed wiringboard formed of a plurality of elements which are laminated together toform a printed wiring board having Z-axis interconnections.

BACKGROUND OF THE INVENTION

Printed wiring boards are conventionally made up of a plurality ofindividual elements joined together to provide various levels of wiringon the surfaces of the elements and interconnections between the variouswiring levels, such interconnection between the various levels oftenbeing referred to as Z-axis interconnections. In some conventionaltechniques for forming such interconnections in the Z-axis, a drillingoperation is required after the various elements have been joinedtogether. This requires precise alignment of all of the elements as wellas precise drilling of the final structure which creates the possibilityof misalignment at least requiring either rework of the board or at mostscrapping of the board after it reaches this late assembly stage. Thus,it is desirable to provide elements for forming a printed wiring boardand a technique for forming the elements in the printed wiring boardwhich does not require drilling in the final stage but, rather, allowsthe individual elements to be formed with the components of the Z-axisconnection which, when finally joined together, will provide thenecessary connection between various layers of metal wiring.

SUMMARY OF THE INVENTION

According to the present invention, a method of forming a core memberfor a composite wiring board and a method of forming the compositewiring board, as well as the core member of the composite wiring boardand the wiring board, are provided. The core member is formed byproviding a dielectric substrate having opposite faces and forming anelectrically conductive coating on at least one face thereof, preferablyby laminating copper on the at least one face. At least one opening isformed through the substrate extending from one face to the other andthrough each conductive coating. An electrically conductive material isdispensed in each of the openings extending through the conductingcoating. At least a portion of the surface of the conductive coating onat least one face is removed to allow a nub of the conductive materialto extend above the substrate face and any remaining conductive materialto thereby form a core that can be electrically joined face-to-face witha second core member or other circuitized structure. In one embodiment,a second core is then formed in a similar manner and the two coresjoined face to face to provide a printed wiring board with electricalinterconnections in the Z-axis, i.e. between the circuit traces onopposite faces of the circuit board so formed. In another embodiment,the core is used to join with at least one other circuitized member. Theinvention also contemplates a core member formed according to thisinvention and a printed wiring board formed using at least one coremember.

DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 show a longitudinal, sectional view, somewhat diagrammatic, ofthe steps to form a core member according to one embodiment of thepresent invention;

FIGS. 7 and 8 show the steps of laminating two core members together toform a printed wiring board according to one embodiment of theinvention;

FIG. 9 shows an another starting material for a core member;

FIGS. 10 and 11 show another technique of forming core members forjoining together;

FIGS. 12 and 13 show two core members formed according to the techniqueshown in FIGS. 10 and 11 joined together;

FIG. 14 shows another embodiment of a bonding film; and

FIGS. 15 and 16 show another embodiment of a core joining twocircuitized structures to form a printed wiring board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and, for the present, to FIGS. 1-6, thesuccessive steps in forming a core member 10 for use in laminating toanother core member to form a printed wiring board according to oneembodiment of the invention are shown. As can be seen in FIG. 1, thecore member 10 includes a dielectric substrate 12 which has metalcoatings 14 and 16 on opposite faces thereof. Dielectric substrate 12can be any conventional dielectric, such as FR4 (a glass reinforcedepoxy), polyimide, polytetrafluroroethylene or other suitable well knowndielectrics. In the embodiment shown in FIGS. 1-6, the metal coatings 14and 16 preferably are copper and, typically, the layer is either oneounce copper (35 um thick) or two ounce copper (70 um thick). However,other thicknesses of copper coatings can be used.

As shown in FIG. 2, a plurality of holes, two of which are shown at 18,are drilled entirely through the substrate 12 and the metal coatings 14and 16. Optionally, the holes 18 may be plated with a metal 20, as shownin FIG. 3, to provide a conductive path between the coatings 14 and 16.However, with good and complete filling of conductive material, as willbe explained presently, the plating 20 may be partially or fullyomitted. Plating 20 can be any type of conventional plating to providethe openings with such metal, such as electroless or electrolyticplating.

As shown in FIG. 4, the openings 18 are filled with a conductivematerial 24. Filling can be done by screening, stenciling, floodcoating, doctor blading, immersing or injecting. In some case, it may bedesirable to heat the adhesive to enhance its flow characteristics and,in some cases, multiple passes might be required to achieve a completefill of the holes 18. Suitable fill materials include conductivepolymers and polymers filled with conductive particles, such as solder,copper particles, silver particles or plated filler particles ormixtures thereof. The polymer can be thermoset or thermoplastic and canbe thinned with a solvent if required. A preferred adhesive material 24is a conductive epoxy sold by the Ablestik Corporation under thetrademark Ablebond 8175 which is a silver filled thermosetting epoxy.Following the filling of the holes as shown in FIG. 4, the epoxy isB-staged, which entails heating the material to a temperature of about130° C. until the degree of cure of the adhesive is advanced to fromabout 20% to about 80% of complete cure. As will become apparent later,the fill material 24 should not be fully cured at this state since itwill be used to adhere to another conductive epoxy in another coreelement to form the printed wiring board, all of which will be describedpresently. If a complete fill of the conductive material results fromthe filling process, the plating 18 may be omitted. However, platinggenerally increases the area of contact, increasing reliability andlowering the electrical contact (resistance) and, therefore, ispreferred.

It should be noted that if some residue of the adhesive material 24remains on the surface of the conductive layers 14 or 16 after fillingthe holes and after it is B-staged, it can be easily removed by chemicalor mechanical polishing processes as are well known in the art. Itshould be noted that, at this point in the processing, no masks havebeen required for filling the holes so there are no registrationsrequired, and any film material that may accidentally remain on thesurface of the metal can be easily removed by polishing or otheroperations.

As seen in FIG. 5, layers of material 14 and 16 are partially etchedaway to form thinned or circuitized conductive surfaces 26 and 28. Suchpartial etching can be performed by the technique described in U.S.patent application Ser. No. 08/968,988, filed Nov. 12, 1997, for“Printed Circuit Board with Continuous Connective Bumps”. Thus, thistechnique of etching around the protrusions 24 by “cold” cupric chlorideetching provides the ideal uniform height of the fill material 24 abovethe etched surfaces 26 and 28 on opposite sides of the dielectricmaterial 12. It is important to note that there are several advantagesto the element in the form that it is in, as shown in FIG. 5. The veryuniform height of the resulting protrusions of the fill material 24 is aparticular advantage. Moreover, there is no fill adhesive residue on themetal surface, and no screening operation is required to form aprotruding bump of adhesive.

Following the general selective etching of the surfaces 26 and 28, thecore is personalized as shown in FIG. 6. In this portion of thepersonalization, the material 26 connects between the protrusions 24 onone face thereof, whereas the material 28 has been entirely removed fromthe other face. This personalization preferably is done by knownphotolithographic processes using either positive or negativephotoresist materials. At this point, the core element as shown in FIG.6 is ready to be laminated with at least one other core element to forma printed wiring structure. It is to be understood that several coreelements typically will be laminated together but the invention is beingillustrated using only two core elements, as shown in FIGS. 7 and 8.

As can be seen in FIG. 7, two core elements 10 a and 10 b are providedwhich are to be laminated together. It will be noted that the two coreelements 10 a and 10 b are very similar except that the personalizationon each of them is slightly different in that on the core element 10 athe personalization 26 a extends from one fill material 24 a to anotherfill material 24 a, whereas on element 10 b the personalization 26 bextends in opposite directions from each of the fill materials 24 b. Asseen in FIG. 7, a pre-drilled bonding film 30, such as the film soldunder the trademark Pyralux LF by Pyralux Corporation, is interposedbetween the two cores 10 a and 10 b. The film 30 has openings 32 drilledtherein which are positioned to align with the conductive fill material24 a, 24 b in the two core elements 10 a and 10 b. Heat and pressure areapplied as represented by the arrows in FIG. 8 to cause the two coremembers to bond together, with the Pyralux LF film acting as an adhesivebond material. Also, the fill material 24 a and 24 b in each of theopenings in the two core members 10 a and 10 b will bond together, asshown in FIG. 8, to form a continuous Z-axis electrical connectionbetween the personalization 26 a on the core element 10 a and thepersonalization 26 b on the core element 10 b. The lamination processalso advances the cure of the conductive fill material 24 a and 24 bpast 80% to the fully cured stage. (Alternatively, the film material 30could be a dry film adhesive which is B-staged and used to laminate thecore elements 10 a and 10 b together.)

FIG. 9 shows the starting configuration of a slightly differentembodiment for a core element 10 c. In this embodiment, the dielectricmember 12 is coated with a first metal, such as copper 40 on one faceand the same metal 42 on the other face, and a second metal 44 on top ofthe metal 40, and metal 46 on top of the metal 42, which is differentfrom copper. Then, after drilling and optionally plating the drilledholes and filling the holes with conductive material, the metal 44 and46 is selectively etched, leaving the metal 40, and which would providethe same configuration as shown in FIG. 5. The metal 44, 46 could becopper, chromium or copper-invar-copper, as well as others, which can beetched by standard etching processes.

FIGS. 10-14 show another embodiment or technique for joining two coreelements together to form a printed wiring board. In this technique, twocores 10 a and 10 b are provided, but instead of a separate dry film 30interposed between the two members to form a laminate structure, adielectric adhesive material 48 a and 48 b is applied to the face of atleast one, and preferably both, of the core elements 10 a and 10 b. Theadhesive material, which is shown, preferably is a thermoset epoxy suchas Epoxy 240 sold by the Dexter Corporation. After the epoxy film 48 bis coated onto a face, as shown in FIG. 10, it is removed from thesurface of the conductive material 24 b, as shown in FIG. 11. This ispreferably by a planar polishing technique. The same technique isperformed on core element 10 a. The two core elements 10 a and 10 b,with the epoxy films 48 a and 48 b coated thereon, as shown in FIG. 12,are then laminated together as shown in FIG. 13. The laminate structureis very similar to that shown in FIG. 8 with the two epoxy films 48 aand 48 b fusing together as film 48.

In still another embodiment, the bonding film 30 takes the form ofanother multi-layer structure, such as a conductive sheet of material 50having an adhesive coating, such as an epoxy 52 coated thereon, andB-staged, as shown in FIG. 14. The conductive element 50 preferably hasholes 54 drilled therein to correspond in location to the conductivefill materials 24 a and 24 b so that it can be laminated. The epoxycoating 52 on the conductive element 50 can be eliminated in one or moreopenings 54, thus providing contact with the conductive element 50 bythe conductive material 24 a, 24 b in the cores 10 a or 10 b (not shownin this Figure).

FIGS. 15 and 16 show yet another technique of forming a core member 10and using the core member to join two circuitized structures to form aprinted wiring board. The core member 10 is very similar in constructionand technique for forming to that shown in FIGS. 1-6, except that, inthis embodiment, all of the metal coatings 14 and 16 on both faces ofthe substrate are removed so that there is a substrate 12 having holes18 therein, which holes are plated with conductive metal 20 just as inthe embodiments shown in FIGS. 1-6. The conductive material 24 disposedin the holes 18 extends above both faces of the substrate 12. This core10 is then used to join two additional wiring circuitized structures 60and 62. These two wiring structures 60 and 62 are formed in a mannervery similar to the formation of the core 10 except that, instead ofpartially removing the metal coatings 14 and 16 to provide thinnedcoating materials 24 and 26 as with the embodiment shown in FIGS. 1-6,the opposite sides of the core members 60 and 62 are personalized sothat the conductive epoxy remains at the same height as the coppercoatings 14 and 16. These coatings 14 and 16 are then personalized inthe core 60 to provide a circuit trace 64 on one face of the core 60 anda pair of bonding pads 66 on the opposite face of the core 60. On thecore 62, the metal coatings 14 and 16 are personalized on opposite facesthereof to form pads 68 on one face of the substrate and pads 70 on theother face.

As shown in FIG. 16, the core 10 is disposed between the two circuitizedmembers 60 and 62, then laminated by heat and pressure as in theprevious embodiment. This will provide a printed wiring board structurehaving continuity between points a and b on one side of the boardthrough the circuitization 64 on the opposite side of the printed wiringboard.

It is to be understood, of course, that the hole in the core 10 and inthe circuitized structures 60 and 62 may be formed without plating ofconductive material thereon, and that the techniques shown in FIGS.10-13 could be used for joining the core to the circuitized structures60 and 62.

This invention permits more direct wiring and avoids the need to runadditional wiring to a local plated through hole to adjacent layers.Avoiding a plated through hole also permits the connections to bedistributed over the XY plane, eliminating concentration of stress andcracks between the plated through hole and the inner planes.

A plated through hole tends to be a rigid structure that connects thetop and bottom of the composite to the internal planes. The structure ofthis invention avoids this and uses materials that are generally morecompliant. Data has been recorded that shows enhanced reliability duringcurrent induced thermal cycles (CITC) testing. Not only is compositedrilling not required as in prior art conventional boards, desmear (toswell and remove drill waste) and plating to form the connectionsbetween the layers is eliminated.

While the invention has been described in conjunction with embodimentsthereof, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art in light of theforegoing teachings. Accordingly, the invention is intended to embraceall such alternatives, modifications and variations as fall within thespirit and scope of the appended claims.

What is claimed is:
 1. A core member for joining to at least oneadditional structure to form a wiring board composite comprising: adielectric substrate having first and second opposite faces and at leastone through opening extending between said opposite faces; electricallyconductive material disposed in each of said openings having a nubextending beyond the opposite faces to thereby form an extendedconductive material for electrically connecting to another core member,said electrically conductive material being sufficiently pliable to bondto similar electrically conductive material in another core member. 2.The invention as defined in claim 1 wherein said electrically conductivematerial is an electrically conductive adhesive.
 3. The invention asdefined in claim 1 wherein said electrically conductive material is afilled thermoset or thermoplastic polymer.
 4. The invention as definedin claim 2 wherein said electrically conductive adhesive is a filledpolymer.
 5. The invention as defined in claim 3 wherein the polymer is athermoset and is cured to between about 20% and about 80% of completecure.
 6. The invention as defined in claim 4 wherein the adhesive is afilled epoxy.
 7. The invention as defined in claim 1 wherein saidthrough openings include conductive material plated on the surfacesthereof.
 8. The invention as defined in claim 1 wherein conductivetraces are formed on at least one face thereof.
 9. The invention asdefined in claim 8 wherein the nub of said electrically conductivematerial extends beyond the conductive traces.
 10. The invention asdefined in claim 9 wherein the nub of said electrically conductivematerial contacts said electrically conductive traces.
 11. The inventionas defined in claim 9 wherein said conductive material is anelectrically conductive adhesive.
 12. The invention as defined in claim11 wherein said electrically conductive adhesive is a filled polymer.13. The invention as defined in claim 12 wherein the polymer is athermoset and is cured to between about 20% and about 80% of completecure.
 14. The invention as defined in claim 12 wherein the adhesive is afilled epoxy.